Problem with link posts to category template

I have this code in my “Single.php”:

<?php
if ( in_category('3') ) {
include 'single_article.php';
}
elseif ('category_parent=9') {
include 'single_teacher.php';
}
else {
include (TEMPLATEPATH . '/index.php');
}
?>

And it work’s great to make every post under category3
use “single_article.php” and every post under the category 9 or under one of it’s subcategories use the “single_teacher.php”.

But when I try to add another template for posts
under category 15 or it’s subcategories like that:

<?php
if ( in_category('3') ) {
include 'single_article.php';
}
elseif ('category_parent=9') {
include 'single_teacher.php';
}
elseif ('category_parent=15') {
include 'videolessons.php';
}
else {
include (TEMPLATEPATH . '/index.php');
}
?>

It’s just take the “single_teacher.php” even
for the posts under category 15.

Any Idea?